common/admin_socket: add a command to raise a signal

common/admin_socket: add a command to raise a signal

The new command "raise <signal> [--after X]" accepts signals
in the forms: '9', '-9', 'kill', '-KILL'
When --after is specified, the program will fork to wait for the timeout

The forked instance will bail out if it detects that the parent PID has changed
which would indicate that the original parent has terminated.

Forking an instance allows to schedule delivery of signals even if the original
process is suspended, e.g.:

  ceph tell mds.a raise CONT --after 10
  ceph tell mds.a raise STOP

Signed-off-by: Leonid Usov <leonid.usov@ibm.com>
Fixes: https://tracker.ceph.com/issues/62882